A reflex or “red dot” sight superimposes a reticle, such as a simple red dot, on a typically unmagnified target. The advantage of a reflex sight is that it is theoretically parallax free, can be held at any distance from the eye, and can be used with both eyes open. Accordingly, the shooter may acquire a target without first carefully placing his eye on an eye-piece, closing the non-aiming eye and finding the target in a limited sight field-of-view. This permits a short range shooter to acquire a target far more rapidly than he could if looking through a telescopic sight.
At longer ranges (e.g. greater than 100 yards) it becomes necessary to use a telescopic sight. Heretofore the problem of installing both a reflex and a telescopic sight on the same gun has not been entirely solved, with suggested solutions sacrificing at least some optical qualities or user convenience.
From a more technical perspective, a reflex sight collimates the light from a luminous reticle and superimposes this light onto a view-window. This places the reticle at an infinite range and virtually eliminates the effects of parallax, when viewing a target that is effectively at an infinite range. Frequently the collimation is performed by a curved mirror that is placed to the side of the path of the light passing through the view-window. Unfortunately, the need to redirect the collimated light reflecting from the curved mirror so that it is superimposed on the view-window complicates the design and tends to reduce performance.